1. Field of the Invention
The present invention relates to an disc and an optical head for optically recording and/or reproducing information on and from the optical disc by radiating a beam of semiconductor laser light to the optical disc.
2. Description of the Prior Art
As an optical head system used in an optical recording and/or reproducing apparatus for recording or reproducing information on and from a recording-medium such as, for example, an optical disc, with the use of a semiconductor laser, such a system as shown in FIG. 12 of the accompanying drawings is well known. Referring to FIG. 12 showing a side view of the prior art optical head system, rays of light emitted from a semiconductor laser 1 are converted by a collimator lens 2 into a collimated laser beam 3. This collimated laser beam 3 enters a polarized beam splitter 4 in the form of P-polarized light and is, after having passed through a 1/4 wavelength plate 5, deflected by a mirror 6 so as to travel towards an optical disc 8 in a direction substantially perpendicular thereto. The laser beam is, before it impinges upon the optical disc 8, focused by an objective lens 7 to form a light spot 9 which is subsequently projected onto the optical disc 8.
A laser beam 10 reflected from the optical disc 8 travels towards the 1/4 wavelength plate 5 through the objective lens 7 and then is reflected the mirror 6. As the laser beam 10 passes through the 1/4 wavelength plate 5, it is transformed into an S-polarized beam which is subsequently reflected by the polarized beam splitter 4 so as to travel towards a photodetector 13 after having passed through a sensor lens 11 and then converged by a cylindrical lens 12. The photodetector 13 detects not only a reproduced signal, reproduced from the optical disc, but also a focusing signal and a tracking signal in a known manner according to a well known anastigmatism and a well known push-pull method, respectively. The focusing signal is applied to a lens drive means 14 through a focusing servo circuit (not shown) to drive the objective lens 7 in a focusing direction (in a direction perpendicular to the optical disc) for a focus adjustment. 0n the other hand, the tracking signal is applied to the lens drive means 14 through a tracking servo circuit (not shown) to drive the objective lens 7 in a tracking direction (in a direction radially of the optical disc) for a tracking error compensation. The mirror 6, the objective lens 7 and the lens drive means 14 are mounted on a carriage 15 so that they can be moved in a direction radially of the optical disc in unison with each other to effect a recording or reproduction of information on or from an arbitrary one of tracks on the optical disc.
In general, the optical disc currently used for recording or reproducing information thereon or therefrom has first and second disc surfaces opposite to each other, both of said disc surfaces being utilizable for recording or reproducing information thereon or therefrom. Not only may information be recorded or reproduced from the disc surfaces one at a time, but simultaneous recording and/or reproduction of information on and/or from the double-sided optical disc may also be possible. This type of optical disc is referred to as a double-sided optical disc in contrast to a single-sided optical disc in which information is recorded or reproduced on or from only one disc surface thereof.
However, with the prior art optical head system of the above described construction, information on only one of the opposite disc surfaces of the double-sided optical disc can be recorded or reproduced at a time and, therefore, the optical disc is required to be removed from the recording and/or reproducing apparatus and then to be turned either first or second disc surface down each time information on the second or first disc surface has been reproduced or recorded. Accordingly, the capacity (the on-line capacity) of the optical disc the end user can enjoy concerning a recording or reproduction of information with respect to a single optical disc with no need to wait for a length of time during which the optical disc is turned over is limited to that of a single-sided optical disc.
One may consider the possibility of disposing two optical head systems, each being of the construction shown in FIG. 12, on respective sides of the optical disc so that information can be recorded or reproduced on or from the respective first and second disc surfaces with no need to turn the optical disc over. However, the mere use of the two optical head systems in one optical recording and/or reproducing apparatus brings about a technically difficult problem.
Specifically, when the double-sided optical disc is turned in one direction for information recording or reproduction, the two optical head systems disposed so as to confront the respective first and second disc surfaces "read" the double-sided optical disc as if it were driven in respective directions opposite to each other. Therefore, it may occur that, during the simultaneous reproduction of information from the opposite disc surfaces of the double-sided optical disc, one optical head system would pick up address signals, recorded on one disc surface, in a correct sequence, but the other optical head system would pick up address signals in a reverse sequence. Thus, even though the apparatus using the two optical head systems may be assembled successfully, an electric circuit system used in the apparatus could make use of a set of address signals from only one of the disc surfaces of the double-sided optical disc at a time.
Moreover, the use of the two optical heads in one apparatus renders the latter complicated in structure and bulky in size.